Siblings have been shown to differ from each other in most measures of mental health (Dunn & Plomin, 1990). This important finding suggests that researchers need to examine factors that vary within families and lead to differences between siblings. Marital discord has been associated with a number of socioemotional developmental outcomes in children. The proposed study will make a unique contribution to research in this area by investigating the differential impact of marital discord on siblings in the same family. One hundred and forty families will be studied longitudinally to follow siblings from childhood to adolescence. Data will include observations of family interaction and self-report measures from family members and children's teachers. The first goal aim is to examine associations between marital discord and individual differences in each siblings' adjustment. Children's individual characteristics (age, birth order, gender, temperament, social competence, and coping skills), their exposure to and understanding of marital conflict, and characteristics of their relationships with mothers and fathers are hypothesized to be associated with variations in their adjustment. Studying more than one child in each family should substantially increase our ability to understand processes connecting marital discord and children's adjustment, as the effects of marital discord are expected to vary from child to child. The second goal is to investigate associations between marital conflict and characteristics of children's relationships with other children. Marital discord is predicted to be linked to disrupted sibling relationships and friendships and peer rejection. Families will be studied longitudinally in order to examine stability and change in associations between marital discord and children's adjustment and relationships. Marital discord is predicted to have both concurrent and cumulative links to siblings' adjustment and relationships. Developmental differences in associations between discord and children's adjustment and relationship will be examined. A unique strength of the longitudinal sibling design is that the effects of development and birth order can be separated by comparing associations between marital discord and siblings' adjustment at the same time, when siblings are different ages, and at different times, when siblings are the same age. Middle childhood and early adolescence are particularly salient developmental periods in which to study the precursors and onset of adjustment problems.